1. The Field of the Invention
The present invention relates to a power supply management system that controls distribution of electric power to a plurality of electric loads installed in a motor vehicle, and in particular, to a power supply management system that limits supply of the electric power to one of the electric loads to reduce consumption of the electric power by the electric load.
2. Description of the Related Art
Recently, the electric loads of motor vehicles have been continuously increasing, because improvement of fuel economy of internal combustion engines in the motor vehicles and purification of exhaust gas emitted from the motor vehicles have been required from an environmental point of view, and electric equipment has replaced mechanical equipment, and new additional electric equipment, for example, navigation systems, and the like, have been installed in motor vehicles.
At the same time, some measures to cope with these situations have been proposed. For example, output electric currents generated by vehicle-mounted generators have been increased, and generation efficiency of the vehicle-mounted generators has been improved by utilizing 42 volt electrical systems. However, when a plurality of electrical equipments which consume a large electric power are simultaneously operated, it is hard to avoid lowering output voltage generated by the vehicle-mounted generators due to a shortage of output electric power generated by the vehicle-mounted generators.
A known power supply system that prevents output voltage generated by the vehicle-mounted generators from lowering is disclosed by Amano et al. in Japanese Patent Application Laid-Open No. 2004-194364. The power supply system of Amano et al. includes a power supply that includes an electric generator and a battery, and a power control unit that controls supply of electric power to an electric load from the power supply. The power control unit includes battery status sensing means for sensing the condition of the battery and load status sensing means for sensing the operating condition of the electric load, and has a function in which the variation of the output voltage of the electric power supplied from the power supply at the time when an operative requirement of the electric load is received by the power control unit is anticipated based on the condition of the battery and the operating condition of the electric load, and the electric current flowing from the power supply into the electric load is limited when the anticipated output voltage of the electric power is smaller than a predetermined value.
While the power supply system of Amano et al. includes a technique that limits the electric current flowing into the electric load taking into consideration of a level of the priority of the electric load and the electric current required to operate the electric load, the total balance of functions that are normally operated in the motor vehicle is not taken into consideration. In other words, when the available electric current is limited, loss of the total balance of functions in the motor vehicle, for example, deterioration of the normal functions of the motor vehicle, may occur. Further, the battery status sensing means for sensing the condition of the battery and the load status sensing means for sensing the operating condition of the electric load provided in the power supply system make the structure of the power supply system complex and result in a cost disadvantage that causes a rise in the manufacturing cost of the power supply system.